Geodetic Concepts

1
Geodetic Concepts
Image from NASA Image Library
2
Reliable Positioning needs

• Recorded positions must be repeatable
• Positions recorded over time and by different
  people must be accurately combined
• Creation of 2 dimensional maps

3
Positioning requirements

• Known starting point
• Measurement units
• Orientation of the measuring system
• Method of displaying measured points

4
Geodetic techniques

• Standard Datum - a geometric model to describe
  the shape of the earth
• Coordinate System - a means of locating points on
  a standard datum
• Projection - conversion of a 3 dimensional world
  to a 2 dimensional map

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Earth as an Ellipsoid
N
a
Equator
b
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Geoid Models

• NAD27 - North American Datum used prior to 1980s
• NAD83 - North American Datum used since 1980s

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Earth Coordinate Systems

• Many coordinate systems have been used over time
• Conversion between coordinate systems provided by
  most GIS packages

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Earth Ellipsoid Coordinates
z
?
?
y
x
? - longitude
? - latitude
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Reference Planes
Source http//www.colorado.edu/geography/gcraft/
notes/coordsys/coordsys.html
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Dr. Munsters sons in Greenwich England. One is
in the eastern hemisphere and the other in the
west. The Prime Meridian runs through the
primary transit of the Royal Observatory.
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Map Projections

• Standard methods of projecting a portion of the
  earths sphere onto flat surfaces (maps) are
  needed
• Lambertian is simple but inaccurate for large
  areas

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Lambertian Projection
Earth projected as a cylinder
13
Universal Transverse Mercator (UTM)

• Earth divided into 60 zones of 6 degrees
  longitude each, counting east for the 180
  longitude meridian
• Each zone has a central meridian as a reference
  point

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Universal Transverse Mercator
180
Longitude
174
186
162
198
Zones
1 2 3 4 5 6 7 ...
54 55 56 57 58 59 60
15
State Plane Projections

• Individual states have defined their own
  projection systems
• These can be converted to UTM with standard
  techniques

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GPS Reference System

• Uses WGS-84 and NAD-83
• Origin is geo-center of earth
• Receivers can do coordinate transformations with
  selected projections